On Tue, 24 Nov 2015 12:21:12 -0700, Don Y
wrote:

How/why do you use these on a swimming pool or solar (water heater?)?

There is an auto / positive drain (2 Inch dia.) at the peak of the
solar panel. It ensures the panel drains completely during winter. It
does freeze hard some years here in the desert. The panel water drains
back into the pool. Not for pool draining.